The proposed research is a combined genetic and biochemical study of mitochondrial biogenesis in Neurospora with the long range goals of defining the roles of the nuclear and mitochondrial genetic systems and understanding how these roles are coordinated and modulated. Specific aims focus on mitochondrial ribosome assembly. We are studying mutants with defects in mitochondrial ribosome assembly in order to identify specific lesions in mitochondrial rRNAs and proteins and to correlate these with defects in mitochondrial ribosome assembly and protein synthesis. In addition, we will attempt to identify mutants with defects in regulatory proteins which coordinate the two genetic systems during the assembly process. A number of specific goals relate to poky (mi-1), an extra-nuclear mutant with an assembly defect leading to a deficiency of mitochondrial (S-5, apparent Mw 52,000) which is synthesized intra-mitochondrially and which may be the site of the primary defect in poky. Major objectives of the proposed research are to continue efforts to identify the primary defect in poky and to determine the role of S-5 in mitochondrial ribosome assembly and protein synthesis.